Evolution of the semiconductor manufacturing industry is placing greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink, yet the industry needs to decrease time for achieving high-yield, high-value production. Minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for a semiconductor manufacturer.
Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology may be used during semiconductor manufacturing to take various measurements of, for example, a semiconductor wafer or reticle. Metrology tools can be used to measure structural and material characteristics associated with various semiconductor fabrication processes. For example, the metrology tools can measure material composition or can measure dimensional characteristics of structures and films such as film thickness, critical dimension (CD) of structures, or overlay. These measurements are used to facilitate process controls and/or yield efficiencies during the manufacture of semiconductor dies.
As semiconductor device pattern dimensions continue to shrink, smaller metrology targets are often required. Furthermore, the requirements for measurement accuracy and matching to actual device characteristics increase the need for device-like targets as well as in-die and even on-device measurements. Various metrology implementations have been proposed to achieve that goal. For example, focused beam ellipsometry based on primarily reflective optics has been proposed. Apodizers can be used to mitigate the effects of optical diffraction causing the spread of the illumination spot beyond the size defined by geometric optics. The use of high-numerical-aperture tools with simultaneous multiple angle-of-incidence illumination is another way to achieve small-target capability.
Other measurement examples may include measuring the composition of one or more layers of the semiconductor stack, measuring certain defects on (or within) the wafer, and measuring the amount of photolithographic radiation exposed to the wafer. In some cases, a metrology tool and algorithm may be configured for measuring non-periodic targets.
FIG. 1 is a schematic showing the collection lens and focus lens configuration of an exemplary ellipsometer. The design of FIG. 1 has high amounts of aberrations. The spot may need to be as small as possible, but aberrations increase the size of the focused spot.
FIG. 1 illustrates an off-axis design. The off-axis design eliminates symmetry in the system. Symmetrical systems tend to be easier to calibrate, and the symmetry allows for assumptions that can make calculations faster. The off-axis design has angles of incidence (AOI) that are higher than an on-axis design of the same numerical aperture (NA). When light reflects at non-normal incidence, it changes the polarization. Having the optics change the polarization state of the light may be a problem because the system of FIG. 1 is used to measure polarization.
FIG. 2 shows how the polarization changes as the angle of incidence changes. The magnitude of the change varies with wavelength, but the basic functional form is quadratic. In other words, if the AOI is reduced by 2×, the polarization change will be reduced by 4×.
Besides the angles of incidence, the lens and mirrors also can affect the polarization. Different rays can have different polarization shifts, which can make calibration challenging. Calibration can lower the accuracy and/or precision of a tool whereas a more accurate polarization can provide better measurements.
A Schwarzschild lens is an objective lens with two spherical mirrors. An example is illustrated in FIG. 3 with a ray trace. For the same NA, the maximum angles of incidence are smaller than the design of FIG. 1. Given the quadratic nature of the polarization shift versus AOI, this is an improvement over FIG. 1. The Schwarzschild lens is rotationally symmetric. As a result of the rotational symmetry, simulations and analysis using the Schwarzschild design require less computing time, and the calibration is simpler.
However, the Schwarzschild lens has limitations. The central obscuration and the legs will block about 25% of the light going through the lens. The central obscuration and the legs also cause extra diffraction relative to the design of FIG. 1. This increases the spot size for a given NA. The effects of the central obscuration and legs have on encircled energy is shown in FIG. 4.
FIG. 4 shows that, to get the same encircled energy as the existing design, the NA needs to be increased by 1.85×. Increasing the NA that much also increases the maximum AOI. As a result, increasing the NA that much almost eliminates the AOI advantage gained by going to an on-axis design.
While the Schwarzschild lens of FIG. 3 is better than the design of FIG. 1, the improvements tend to be small. Increasing the NA by 1.85× can cause other problems that, while different, can be just as detrimental to the system performance.
Therefore, what is needed is an improved lens system that addresses the limitations of the Schwarzschild lens.